The present invention relates generally to reversible proportioning valves and, more particularly, to a reversible proportioning valve apparatus and method for controlling pressure without the use of external power.
A variety of reversible proportioning valves have been proposed. Typically, such a valve has a piston cooperating with an internal valve which is normally open to connect the inlet and outlet of the valve, but is closed in response to movement of the piston due to an increase in pressure at the inlet of the valve.
In U.S. Pat. No. 4,319,786, issued to Nogami, a "ball on ramp" valve is used to control the "knee" of the pressure curve. In the brake pressure control device of Nogami, a partition member is assembled within the valve chamber to ensure the precise initial setting of the valve element and is provided with a fluid passage to ensure the optimum control of fluid flow acting on the valve element. However, this type of valve is susceptible to vibrations as well as longitudinal deceleration and, accordingly, has not been successful.
Another reversible proportioning valve is disclosed in U.S. Pat. No. 3,479,095, issued to Lewis et al. The Lewis et al patent discloses a reverse proportioning valve between a pressure generating device and a brake cylinder. The reverse proportioning valve has pressure responsive means with a different area exposed to the actuating pressure than that which is exposed to the pressure of the brake cylinder controlling a tilt valve regulating the bypass of fluid through the reverse proportioning valve. However, the "tilt valve", which is essential to the operation of the Lewis et al proportioning valve, is difficult to execute.
Finally, U.S. Pat. No. 3,449,018, issued to Hales, discloses hydraulic brake control valves having a piston controlling operating movement of an internal valve for cutting off the inlet from the outlet of the control valve. The hydraulic control valves of Hales appear to be reverse proportioning valves in which the piston works within a sleeve which is itself slidable in a bore in the valve body. When the inlet pressure increases, the piston is moved relative to the sleeve to close the internal valve, and thus vary the rate at which pressure is transmitted through the control valve. When the inlet pressure is relieved, outlet pressure is quickly relieved by this placement not only of the piston, but also of the sleeve. However, the hydraulic brake control valves of Hales use a very small check ball to control flow. The small passage associated with the small check valve will restrict the brake application rate.
It is seen then that there is a need for a reversible proportioning valve which is straightforward and easily manufactured, which employs a stepped piston/cylinder to control pressure without the use of external power.